1. Field of the Invention
The present invention relates to a liquid crystal display device (LCD), and more particularly, to an LCD constructed to repair its defective cell and a repairing method thereof.
2. Discussion of the Related Art
LCDs have low power consumption and good portability and thus are considered to be a next-generation display device.
Generally, liquid crystal is manufactured as a liquid crystal cell for use in the LCD.
The liquid crystal cell is constructed in such a way that liquid crystal is filled between two glass substrates or between two transparent plastic substrates. Transparent electrodes (common electrode, pixel electrode) are formed on the substrate to apply a voltage to the liquid crystal, thereby controlling the switching on or off of the liquid crystal cell.
More specifically, the light transmittance of the LCD is controlled by a voltage applied to the transparent electrodes, and thus text and an images can be displayed by a light shutter effect.
An active matrix (AM) LCD is provided with a switching element capable of controlling whether or not to apply a voltage to each pixel. The AM LCD is widely used because it can provide high resolution and excellent reproduction of motion pictures and video.
FIG. 1 is a perspective view illustrating a portion of a related art LCD, particularly an active region thereof.
Referring to FIG. 1, upper and lower substrates 110 and 130 are disposed in such a way that they face each other and are spaced apart from each other by a predetermined distance. A liquid crystal layer 150 is interposed between the upper and lower substrates 110 and 130.
A plurality of gate and data lines 132 and 134 are arranged to intersect each other, and a thin film transistor (TFT) T is formed at an intersection of the data and gate lines 132 and 134. A pixel electrode 146 connected to the TFT T is formed in a pixel region P defined by the intersected gate and data lines 132 and 134.
Although not shown in FIG. 1, the TFT T includes a gate electrode supplied with a gate voltage, source/drain electrodes supplied with a data voltage, and a channel for controlling whether or not to apply a voltage by a difference between the gate and data voltages.
A color filter layer 112 and a common electrode 116 are sequentially formed on an inner surface of the upper substrate 110.
The color filter layer 112 includes an R/G/B color filter transmitting only light of a specific wavelength, and a black matrix disposed between the color filter layers to block light in the region where the alignment of liquid crystal is or cannot be controlled.
Upper and lower polarizers 152 and 154 are disposed on outer surfaces of the upper and lower substrates 110 and 130, respectively, to transmit only light parallel to a polarization axis. A backlight unit is disposed under the lower polarizer 154.
The LCD is completely manufactured by performing an array substrate manufacturing process for forming the switching elements and the pixel electrodes, a color filter substrate manufacturing process for forming the color filters and the common electrodes, and a liquid crystal cell process for interposing liquid crystal between the two substrates.
The liquid crystal cell process is roughly divided into an alignment layer forming process for aligning liquid crystal molecules, a cell cutting process, and a liquid crystal forming process. A polarizer is attached on an outer surface of a liquid crystal panel completed through the above processes, and a driving circuit is connected to it, thereby completing the LCD.
FIG. 2 is a view illustrating a related art process of repairing a defective cell of a liquid crystal panel.
Referring to FIG. 2, a plurality of gate and data lines 262 and 274 are formed on a liquid crystal panel 290 in such a way to intersect each other. A TFT T is formed at an intersection of the gate and data lines 262 and 274. A pixel electrode 276 is connected to the TFT T in each pixel.
A dielectric material (not shown) is interposed between the gate line 262 and the pixel electrode 276, thereby forming a storage capacitor Cst.
After the liquid crystal panel 290 is completely manufactured through this liquid crystal cell process, a cell checking process is performed for checking whether or not a defective pixel exists. This is performed by displaying a test pattern on a screen of the liquid crystal panel 290. Thereafter, any detected defective pixels are repaired.
Defects of the liquid crystal panel 290 include a color defect in each pixel, a point defect, and a light defect. The point defect includes a bright point (always-on cell) and a dark point (always-off cell). The light defect is due to disconnection between the drain electrode of TFT and the pixel electrode 276 or the broken channel of a TFT (normally white mode TN).
These defects appear when the test pattern is displayed on the liquid crystal pattern 290. The positions of defective pixels are detected and then the defective pixels are repaired.
FIG. 2 illustrates an exemplary process for repairing a defective cell I, which appears as a bright point due to a broken defect or a signal defect when a black pattern is displayed on the screen, by darkening the bright point.
A pixel electrode 276 of the defective cell I is shorted by laser from the drain electrode of TFT or is shorted by laser between the drain electrode and the source electrode of TFT. Accordingly the defective cell I is darkened to repair the bright point.
A white point A may occur due to an alignment layer printing process or environments.
Also, when the disconnection of the pixel electrode 276 from the gate line 262 and the ITO cutting process is unsuccessfully performed, the gate and data lines 262 and 274 become shorted together, thereby causing the line defect.
As above, there may occur a case where it is impossible to repair the point defect, such as the white point A. In this case, the yield and production of the liquid crystal panel is degraded.